36.1 Characteristic Strength of Materials
The term 'characteristic strength' means that value of the strength of the material below which not more than 5 percent of the test results are expected to fall. The characteristic strength for concrete shall be in accordance with Table 2. Until the relevant Indian Standard Specifications for reinforcing steel are modified to include the concept of characteristic strength, the characteristic value shall be assumed as the minimum yield stress/0.2 percent proof stress specified in the relevant Indian Standard Specifications.
36.2 Characteristic Loads
The term 'characteristic load' means that value of load which has a 95 percent probability of not being exceeded during the life of the structure. Since data are not available to express loads in statistical terms, for the purpose of this standard, dead loads given in IS 875 (Part 1), imposed loads given in IS 875 (Part 2), wind loads given in IS 875 (Part 3), snow load as given in IS 875 (Part 4) and seismic forces given in IS 1893 shall be assumed as the characteristic loads.
36.3 Design Values
36.3.1 Materials
The design strength of the materials, is given by
where
- = characteristic strength of the material (see 36.1), and
f
- partial safety factor appropriate to the material and the limit state being considered.
36.3.2 Loads
The design load, is given by
where
- characteristic load (see 36.2), and
- partial safety factor appropriate to the nature of loading and the limit state being considered.
36.3.3 Consequences of Attaining Limit State
Where the consequences of a structure attaining a limit state are of a serious nature such as huge loss of life and disruption of the economy, higher values for and than those given under 36.4.1 and 36.4.2 may be applied.
36.4 Partial Safety Factors
36.4.1 Partial Safety Factor for Loads
The values of given in Table 18 shall normally be used.
36.4.2 Partial Safety Factor for Mateiral Strength
36.4.2.1
When assessing the strength of a structure or structural member for the limit state of collapse, the values of partial safety factor, should be taken as 1.5 for concrete and 1.15 for steel.
NOTE — γ values are already incorporated in the equations and tables given in this standard for limit state design.
36.4.2.2
When assessing the deflection, the material properties such as modulus of elasticity should be taken as those associated with the characteristic strength of the material.